U.S. patent number 5,124,554 [Application Number 07/656,942] was granted by the patent office on 1992-06-23 for explosives detector.
This patent grant is currently assigned to Rolls-Royce and Associates Limited. Invention is credited to Martin J. Allen, Peter H. Fowler, John D. Rogers, Peter A. E. Stewart.
United States Patent |
5,124,554 |
Fowler , et al. |
June 23, 1992 |
Explosives detector
Abstract
Apparatus for non-invasively inspecting an object, such as an
item of luggage, for explosives material comprises a multi-channel
thermal neutron inspection system having a plurality of neutron
irradiation chambers. Simultaneous operation of several channels
increases maximum system throughout several times. Each chamber has
a lithium neutron source which is stimulated to neutron production
by a proton beam. Beam switching magnets are energized by pulsing
to divert a common proton beam to each source in turn. The initial
beam is generated by a radio frequency quadrupole accelerator. The
advantages of this system are very low residual source activity and
controllable neutron production thereby minimizing safety hazards.
The irradiation chamber may contain several different gamma ray
detectors to identify the presence elements present in explosives
material. In addition a neutron radiography imaging means may be
employed to identify the presence of potential shielding
materials.
Inventors: |
Fowler; Peter H. (Bristol,
GB2), Stewart; Peter A. E. (Bristol, GB2),
Rogers; John D. (Chepstow, GB3), Allen; Martin J.
(Bristol, GB2) |
Assignee: |
Rolls-Royce and Associates
Limited (Derby, GB2)
|
Family
ID: |
26296694 |
Appl.
No.: |
07/656,942 |
Filed: |
February 19, 1991 |
Foreign Application Priority Data
|
|
|
|
|
Feb 20, 1990 [GB] |
|
|
9003851.4 |
Feb 14, 1991 [GB] |
|
|
9103100.5 |
|
Current U.S.
Class: |
250/358.1;
250/390.02; 250/390.04; 376/159 |
Current CPC
Class: |
B64F
1/368 (20130101); G01N 23/222 (20130101); G01V
5/0069 (20161101); G01V 5/0008 (20130101); G21K
5/04 (20130101); H05H 3/04 (20130101); G01N
33/22 (20130101); G01N 2223/0745 (20130101); G01N
2223/204 (20130101); G01N 2223/206 (20130101); G01N
2223/40 (20130101); G01N 2223/501 (20130101); G01N
2223/302 (20130101) |
Current International
Class: |
B64F
1/36 (20060101); B64F 1/00 (20060101); G01V
5/00 (20060101); G01N 23/222 (20060101); G01N
23/22 (20060101); G21K 5/04 (20060101); H05H
3/00 (20060101); H05H 3/04 (20060101); G01N
33/22 (20060101); G01N 023/00 (); G01N
023/05 () |
Field of
Search: |
;250/358.1,390.02,390.04,390.07,359.1 ;376/159 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
0295429 |
|
Dec 1988 |
|
EP |
|
0297249 |
|
Jan 1989 |
|
EP |
|
8702170 |
|
Mar 1988 |
|
WO |
|
1392169 |
|
Apr 1975 |
|
GB |
|
2150737 |
|
Jul 1985 |
|
GB |
|
2151837 |
|
Jul 1985 |
|
GB |
|
2217009 |
|
Oct 1989 |
|
GB |
|
Other References
Foster et al., "Method for Measuring Total Cross Sections with
Neutrons Having Energies from 2.5 MeV to 15 MeV", Nuclear
Instruments & Methods 36, pp. 1-12. .
Gonazi et al., "Explosive Detection System Based on Thermal Neutron
Activation", IEEE AES Magazine, Dec. 1989, pp. 17-20. .
Swinth, Conference, Proceedings of the Society of Photo Optical
Instrumentation; Engineers Seminar on Imaging Techniques for
Testing and Inspection, Los Angeles, Cailf. 1972, pp. 23-30. .
"Neutrons Give Ray of Hope in Halting Plastic Explosive", the
Sunday Times (London, England) 22 Jan. 1989. .
"Neutron Detection for Kennedy", Flight International, 8 Jul. 1989.
.
"Airport Detectors not Ready for Terrorists", New Scientist, 26
Jan. 1991..
|
Primary Examiner: Hannaher; Constantine
Assistant Examiner: Glick; Edward J.
Attorney, Agent or Firm: Oliff & Berridge
Claims
We claim:
1. Apparatus for detecting explosives material in an item by means
of a thermal neutron activation process comprising:
a structure having walls defining an irradiation chamber for
receiving the item,
neutron source means for producing a beam of mixed energy neutrons
including:
a lithium target,
a source of protons,
a radio frequency quadropole accelerator (RFQ) means for
accelerating the protons, the RFQ means being energized to
accelerate the protons to an energy of 2.5 MeV in a direction of
the lithium target whereby an endothermic reaction produces
neutrons in a direction of an incident beam having a mean energy of
0.5 MeV and a peak energy of 0.77 MeV,
means within the irradiation chamber for moderating and reflecting
neutrons to produce a uniform distribution of thermal neutrons
within the item placed within the irradiation chamber,
gamma ray detecting means disposed adjacent the irradiation chamber
to detect gamma rays emitted by contents of the item as a result of
thermal neutron activation caused by said neutrons and to produce
an output indicative of an energy spectrum of aid detected gamma
rays, and
means for collating a plurality of said outputs to detect a
resonance in a gamma ray energy spectrum indicative of a presence
of at least one predetermined element in said item.
2. Apparatus as claimed in claim 1 comprising a plurality of
irradiation chambers each of which is provided with a lithium
target, a beam of protons from a single proton source being
selectively switched to each target as required.
3. Apparatus as claimed in claim 2 wherein the means for
selectively switching the proton beam to a target comprises a beam
switching magnet.
4. Apparatus as claimed in claim 1, wherein the gamma ray detecting
means comprises a plurality of inorganic detectors disposed in a
first plane containing the neutron source means and intersecting a
volume in the irradiation chamber through which the item being
inspected is passed.
5. Apparatus as claimed in claim 4 wherein the plurality of
inorganic gamma ray detectors are arranged in a U-shaped array an
open side of which faces towards the neutron source means.
6. Apparatus as claimed in claim 5 wherein side limbs of the
U-shaped array of inorganic detectors contain a plurality of
inorganic detectors disposed in circular arrays facing inwards
towards the inspection volume.
7. Apparatus as claimed in claim 4 wherein the inorganic detectors
for detecting gamma rays comprise sodium iodide.
8. Apparatus as claimed in claim 4 further comprising additional
gamma ray detecting means of more sensitive type disposed in a
second plane inclined with respect to the first plane.
9. Apparatus as claimed in claim 8 wherein said additional gamma
ray detecting means is positioned to avoid impingement by neutrons
in the inspection beam.
10. Apparatus as claimed in claim 8 wherein the additional gamma
ray detecting means comprises germanium.
11. Apparatus as claimed in claim 11 further comprising neutron
detecting means directly responsive to the impingement of neutrons
disposed relative to the irradiation chamber to detect a neutron
capture profile of an inspected object.
12. Apparatus as claimed in claim 11 wherein the neutron detecting
means comprises a plurality of neutron detectors disposed in a
U-shaped array an open side of which faces towards the neutron
source means.
13. Apparatus as claimed in claim 12 wherein the U-shaped array of
neutron detectors is disposed inside a U-shaped array of gamma ray
detectors.
14. Apparatus as claimed in claim 11 wherein the neutron detectors
comprise cylinders of metal having a low neutron capture
cross-section and filled with helium 3 (He3).
15. Apparatus as claimed in claim 1 wherein the means for collating
the plurality of said outputs includes means for collating the
plurality of said outputs to form a gamma ray energy spectrum,
means for storing said energy spectrum and means responsive to said
stored energy spectrum to compare an amplitude of peaks formed in
the energy spectrum with predetermined threshold limits to
determine a presence of designated elements corresponding to said
peaks.
16. Apparatus as claimed in claim 15 further comprising alarm means
responsive to positive detection of the designated elements to warn
of their detection.
17. Apparatus as claimed in claim 1 further comprising means for
passing the object to undergo inspection through the irradiation
chamber in order to produce detector signals in respect to the
whole volume of the object.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to explosive detectors. In particular the
invention relates to a nuclear technique for monitoring objects
such as luggage and parcel items and to screen such items for the
presence of explosives materials.
The invention concerns a non-invasive method of inspection which
involves subjecting the items to a thermal neutron environment and
observing the gamma capture reaction.
It is an object of the present invention to provide a method and
apparatus for non-invasively inspecting luggage by detecting gamma
radiation emitted by selected elements in response to neutron
irradiation.
The invention also has for an objective to positively detect the
presence of explosives material by identifying certain
characteristic elements. Further objectives include to pre-empt
countermeasures which might be taken to conceal the explosives
material, e.g. shielding; and to minimise false alarms while
ensuring that quantities of material above a predetermined minimum
do not go undetected.
Still further objectives of the system design include minimum
system weight and minimum occupied floor area for the inspection
system, source shielding and the baggage handling system. The
baggage handling system is also intended to be capable of reaching
a throughput of at least 10 bags/minute per channel in a
multi-channel facility.
2. Description of the Prior Art
There are already in existence several types of non-invasive
baggage inspection systems. The most familiar to air travellers is
the ubiquitous x-ray apparatus. This type relies entirely on the
visual skills and vigilance of human operators to spot suspicious
objects. The systems have no inherent capability to detect
explosives material itself.
There are a number of other systems designed to sense explosives
material but each has perceived drawbacks which the present
arrangements seek to either avoid altogether or to improve upon.
The systems considered most effective use neutrons as the
inspection medium. Of these there are three types of source each of
which gives rise to a characteristic problem specifically addressed
by the present invention.
In a first of these systems neutrons are produced by a
deuterium-tritium reaction which takes place inside a containment
vessel, generally referred to as a "reaction tube". These "tubes"
are expensive to replace. Although the source is controllable the
major drawback stems from the energy of the reaction which produces
neutrons in a narrow energy band at 14 MeV only. These neutrons are
too fast for the purpose and have to be moderated. At such energies
shielding has to be very bulky.
A second type of system uses a continuous source, for example a
californium isotope Cf.sup.252. This produces neutrons having a
useful spread of energies in the range 0.5-14 MeV with a mean about
2 MeV. Unfortunately it is a continuously radio-active source. This
is undesirable from a security standpoint. The source is
effectively uncontrolled, and because of the high energy of the
neutrons requires bulky shielding.
The third type of source, also controllable, comprises a radio
frequency quadrupole accelerator, which will be subsequently
referred to as an RFQ accelerator. This impinges a beam of
deuterons onto a beryllium target. It generates a reasonably
monoenergetic neutron beam at about 7 MeV. The neutrons are still
more energetic than required so a considerable degree of shielding
and moderation is still necessary.
The advantages of the present invention which will be apparent from
the subsequent description. The neutrons produced are low energy
reducing shielding requirements. Other advantages includes easy
source control, and the neutron source has only low and soft
residual radioactivity. Also a high neutron flux at low proton
current allows multi-channel inspection to raise total system
throughput.
SUMMARY OF THE INVENTION
According to the broadest aspect of the present invention a method
of detecting the presence of explosives material in an item under
investigation includes the steps of exposing the said item to an
environment of thermal neutrons and detecting gamma rays
characteristic of selected elements.
Preferably the selected elements are nitrogen, carbon and hydrogen
in any combination thereof.
According to one aspect of the invention there is provided
apparatus for detecting explosives material in an item
comprising:
a structure having walls defining an irradiation chamber for
receiving the item,
means for producing a beam of mixed energy neutrons including a
lithium target, a source of protons and means for accelerating the
protons in the direction of the target to produce said
neutrons,
means for producing a uniform distribution of thermal neutrons
within the item,
gamma ray detecting means disposed adjacent the irradiation chamber
to detect gamma rays emitted as result of irradiation and to
produce an output indicative of the energy of a detected gamma ray,
and
means for collating a plurality of said outputs to detect peaks in
a gamma ray energy spectrum indicative of the presence of a
predetermined element or elements in said item.
The uniform distribution of neutrons in the target volume is
achieved by permitting a fraction of primary energy neutrons from
the source, i.e. unmodulated, to travel directly to the item and a
further substantial proportion to reach the target by reflection.
Neutron capture takes place in the energy range 10 mev up to 100
mev (milli-electron volts). To ensure good penetration and uniform
distribution a proportion of the neutrons are moderated to this
level within the item. Others are slowed outside the item so they
reach a thermal energy level at or close to the surface of the
item. Lithium is a most suitable target material for achieving this
because it possesses a high yield of neutrons in an advantageous
primary energy range.
Preferably the apparatus comprises a plurality of irradiation
chambers each of which is provided with a lithium target, and a
beam of protons from a single proton source is selectively switched
to each target as required.
According to another aspect of the invention the means for
producing the neutrons comprises a source of protons, means for
accelerating the protons towards a target, a lithium target
arranged to emit neutrons into the irradiation chamber in response
to protons impinging on the target.
Preferably the means for accelerating the protons comprises a radio
frequency quadrupole accelerator. Also, the irradiation chamber is
preferably at least partially surrounded by neutron reflecting
means arranged to reflect neutrons towards the item whereby to
produce a substantially uniform distribution of neutrons within the
item.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and how it may be carried into practice will now be
described in greater detail with reference to an example
illustrated in the accompanying drawings, in which:
FIG. 1 is an overall perspective of a non-invasive luggage
inspection system partly cutaway to reveal internal features,
FIG. 2 is an enlarged view of a portion of FIG. 1 showing a neutron
source and irradiation chamber in more detail,
FIG. 3 shows a schematic diagram of a vertical section through the
source and chamber of FIG. 2 viewed in the direction of travel of
an item of baggage,
FIG. 4 shows a view corresponding to that of FIG. 3 from the side
of the apparatus,
FIG. 5 shows a diagram plotted graphically of a gamma ray detector
output, and
FIG. 6 shows a schematic diagram of the detector output signal
processing system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a non-invasive inspection system of the type
that may be installed in airports to screen luggage for explosives
material. The inspection equipment is housed within a shielding
enclosure 2, to be described more fully below. Baggage is carried
towards the enclosure 2 in a conveyer system 4 and is similarly
carried away from it by one of two further conveyors 6 and 8. An
item is delivered to one or other of the conveyors 6 or 8 according
to its inspection results. Items on conveyor 6 have passed
inspection and may be stowed ready for travel. Those which fail
inspection are re-routed by a drop-down gate 9 to conveyor 8.
Conveyer 8 delivers these to a designated location where they can
be examined more closely.
The inspection equipment is a multi-channel system served by one
radio frequency quadrupole accelerator. Hereinafter radio frequency
quadrupole will be referred to by its initials RFQ. The illustrated
equipment has four channels A, B, C and D. A greater or lesser
number of inspection channels may be employed according to the
required throughput. An item of incoming baggage is carried by a
horizontal conveyor 4 through the inspection chamber 10 on one of
four conveyor systems 12, 14, 16, 18. Individual items are selected
for each channel merely on the basis of a conveyor vacancy.
The inspection conveyors 12-18 are disposed at right angles to the
main conveyor 4. Each receives an item of luggage through a capture
gate such as at 20. The other channels all have identical
equipment. For the sake of brevity and clarity only one channel is
referenced in the drawings and will be described here.
Capture gate 20 is swung open and across conveyor 4 to entrain an
item of baggage when its associated inspection chamber is vacant. A
captured item first passes onto a set of rollers 22 and then to an
automatic weighing platform 24. After capture has been made the
gate swings shut behind the item. Further items then pass onward to
the next or a subsequent channel, or are recycled if all channels
are temporarily full.
In FIG. 1, channel A is shown in the process of capturing an item.
Channel D is ready to receive another item with its capture gate
open. A previous suitcase has passed into a second inspection
chamber. Chamber C has just captured a bag which is being loaded
onto the automatic weighing platform 24. Its capture gate is
closed. Meanwhile in channel B a suitcase is seen entering a first
inspection chamber.
The inspection apparatus is housed wholly within the walls of
shielding enclosure 2. From the automatic weighing platform 24 the
circulating belt of conveyor 12 carries the luggage item to a first
x-ray inspection chamber 26. The first chamber 26 is defined front
and back, in the direction of luggage travel, by the external front
wall 28 of the enclosure 2 and an internal dividing wall 30. Access
openings 32, 34 formed in these walls through which the conveyor
passes. Radiation proof shutters 36, 38 close these openings
respectively, opening only to pass baggage items into the front and
out through the rear wall of the chamber. At the rear of the
structure an external back wall 39 is spaced from the rear of the
radiography chamber to form a safe exit chamber. Baggage items pass
through further access openings closed by radiation shutters,
operation of which is synchronised to prevent direct line of sight
into the neutron irradiation chamber.
A conventional x-ray radiography unit 40 is positioned in the roof
of the first chamber directed downwards towards the conveyor. When
activated this x-ray unit will produce an x-ray image of any item
positioned beneath it. Operation of the x-ray unit 40 is
synchronised with the shutters and the conveyor. In the particular
example being described x-ray units for adjacent channels, e.g. A
and B, C and D, are located in a common inspection chamber. Thus
adjacent conveyors 12 and 14, also 16 and 18, feed luggage into
common inspection chambers. However, these inspection chambers may,
if preferred, contain only a single x-ray unit. Other multiple
groupings are also possible where suitable.
After x-ray the baggage item is moved from the first, x-ray
inspection chamber 26 into a second, neutron radiography chamber
42. Luggage enters and leaves this second chamber through access
openings 44, 46 in front and rear walls 48, 50 respectively. As
before openings 44, 46 are normally closed by radiation proof
shutters 52, 54. Operation of the shutters is synchronised with
movement of the luggage conveyor 12 together with energisation of
neutron radiography unit. The neutron radiography unit is indicated
generally at 56 in FIG. 1 and will be described in greater detail
with reference to FIG. 2.
Each neutron radiography unit 56 includes a neutron source
comprising a lithium target upon which protons are made to impinge.
The unit 56 is located in the uppermost part of the inspection
chamber 42. In this location it is disposed to direct neutrons
towards the item undergoing inspection. Protons enter the unit via
a short length of vertical beam tube 58 which passes through the
roof and shielding of the chamber. Similarly to the x-ray units the
neutron radiography units are housed in pairs within an inspection
chamber. Preferably as shown in FIG. 2 the radiography units share
the gamma-irradiation chamber. They may, however, be housed singly
or in other multiples. The vertical beam tube 58 to each of the
units receives protons from a common source. Each vertical tube 58
is connected with beam switching magnet 60 placed in line with a
main beam tube 62. For four neutron radiography units in four
channels A, B, C and D four beam switching magnets 60 are provided
in the main beam tube 62. Each beam switching magnet 60 can
selectively switch the proton beam into a beam tube 58 leading
directly to a corresponding radiography unit 56.
A proton source and means for accelerating the protons is located
at one end the main beam tube 62. The output beam tube 64 of an RFQ
accelerator, generally indicated at 66, injects protons into the
tube 62. The detailed construction and operation of the RFQ
accelerator 66 is not part of the subject matter of the present
invention. A more detailed description is available elsewhere in
published literature if required. It will not be further explained
here. In response to a pulsed electrical input at 68 the RFQ
accelerator produces a pulse of 2.5 MeV protons along main beam
tube 64. An appropriately synchronised control pulse to a selected
beam switching magnet 60 will then switch the pulse of protons into
the corresponding branch beam tube 58 to impinge on a neutron
producing target.
Referring now to FIG. 2, the gamma-ray and neutron radiography unit
56 is shown in greater detail. Protons from switching magnet 60
travel down a short vertical beam tube 58 towards a lithium target
72. The consequential reaction produces a neutron flux of mixed
energies generally directed in the downward direction. The angular
distribution of this neutron flux is strongly peaked in the forward
direction. The maximum energy of the produced neutrons is only 0.77
MeV (Mega-electron volts) with a mean energy of 0.5 MeV in the
forward direction. Neutrons at wider angles have an average energy
of only 0.3 MeV and are easily reflected or absorbed by surrounding
shielding 76,78.
The target 72 includes a moderator 73 surrounding the source which
reflects and absorbs wide angle neutrons. The surrounding shielding
absorbs stray neutrons not travelling in the preferred direction
towards the inspection volume. This, in combination with the
strongly peaked angular distribution, allows about 50% of the
neutrons produced to be emitted into the exposure volume. The
neutron beam is indicated by the shaded arrow 74 in the drawing.
The collision involved in each neutron reflection absorbs a
proportion of the neutron's energy. As a result the average energy
of the beam is progressively thermalised.
Immediately below the lithium source and in front of an aperture in
the target shielding is placed a bismuth gamma shield 75 through
which the beam 74 emerges. This shield absorbs high energy gamma
rays emitted by the target which could damage, or at least impair
the performance, of the gamma ray detectors below.
The irradiation volume where an item of baggage is irradiated is
defined by surrounding reflectors 76,78 and shielding indicated at
77, 79 in FIGS. 2, 3 and 4. The circulating belt conveyor 12 passes
through this volume.
It enters and exits the shielded volume through openings protected
by movable radiation shutters, which have been omitted from FIG. 2
for clarity but which may be seen in FIGS. 1 and 4. As is clear
from FIG. 4 the shutters 52, 54 incorporate reflectors and neutron
shielding. In the side walls on either side of the conveyor and set
into the floor beneath it are gamma detector arrays 80, 82 and
84.
The three sets of gamma ray detectors 80,82 and 84 are arranged in
a U-shaped array, the open side of the U-shape facing upwards
towards the neutron source 72. Basically the detectors are arranged
in a vertical plane, which contains the source or target 72 and
which intersects the space through which the item being inspected
is passed.
In the lowest of the three sets of detectors 82 the individual
detectors are arranged in a linear array adjacent the lower surface
of the inspected item. The gamma-ray detectors are protected from
incident neutrons by sufficient neutron shielding, e.g. borated
paraffin wax. The remaining detectors of sets 80 and 84 are
arranged in two circular arrays facing each other across the
inspection space. The item passes between these two arrays. See
FIGS. 3 and 4 which shows views at right angles.
The gamma ray detectors contain inorganic scintillation material
responsive to the incidence of gamma ray energy. The preferred
material is sodium iodide.
However, other possible materials from which the detectors may be
formed include barium flouride, bismuth germanate, etc. The
selected detector material should not be susceptible to damage by
neutrons. In the example being described a layer of shielding is
placed in front of the detectors.
An additional high resolution gamma ray detection means is provided
with an angle of view encompassing substantially the whole of the
inspection space. A single germanium high resolution detector 86 is
positioned in an upper part of the inspection chamber and inclined
at an angle with respect to the inspection plane. High purity
germanium is easily damaged by the fast neutrons in the beam. This
detector is therefore positioned where it can be adequately
shielded from the faster neutrons.
To achieve consistent detection with a high degree of confidence it
is essential to have uniform distribution of neutrons in the right
energy range throughout the volume of the item to be inspected with
it in place. In the present arrangement this distribution is
achieved by a combination of factors. The item is effectively
placed in a "well" formed by the surrounding neutron reflecting
walls. While it is in the "well" it is bathed in thermal neutrons.
Some of the neutrons are thermalised inside the item largely by the
normal hydrogen content of luggage. Others arrive from the
reflecting/moderating walls already thermalised. Some of the
neutrons arrive directly from the source, or after relatively few
collisions, while others have been reflected numerous times. Within
the item, therefore, the distribution neutrons is isotropic with a
substantial energy range.
The most common stable isotope of nitrogen is N.sup.14. The neutron
capture reaction between the thermal neutrons and the bombarded
nitrogen atoms produces N.sup.15 which emits gamma rays. About 15%
of these capture reactions emit gamma-rays with a characteristic
energy of 10.83 MeV. The spectrum of emitted gamma ray energy
therefore exhibits a strong peak at 10.83 MeV. Other elements,
including carbon and hydrogen, behave in a similar fashion and emit
gamma rays with characteristic tell-tale peaks at other
energies.
The scintillation detectors in response to the incidence of a gamma
ray produce an electrical output pulse the amplitude of which is
proportional to the gamma ray energy. FIG. 5 shows in graphical
form typical collated outputs from a sodium iodide detector and a
germanium detector. It will be apparent that the sharper resolution
of the germanium detector permits clear identification of gamma ray
peaks due to its superior resolution. The main sodium iodide
detectors being spaced along three sides of an item under
inspection have useful positional sensitivity for nitrogen and
other elements detected. This is inherent in the distribution with
time of the appropriate gamma ray counts of individual detectors.
The germanium detector is able to act as a general monitor and to
identify suspicious peaks due to the presence of certain materials.
For example, mercury is often present as mercury fulminate in
detonators. Chlorine is the base of an alternative family of
explosives which contain no nitrogen.
A further arrangement of detectors for carrying out neutron
radiography is illustrated in FIGS. 2,3 and 4. This comprises a
plurality of neutron detectors, one of which is indicated at 88.
Such a detector essentially comprises a cylinder of metal which has
a low neutron capture cross-section and which is filled with helium
3 (He3). The detection cylinders 88 are mounted with their
cylindrical axes parallel with the direction of movement of the
item and transverse to the general direction of incident neutrons,
arrow 74. The detectors are also distributed in a U-shaped array on
three sides of an object undergoing inspection. This U-shaped
radiography detector array may be positioned beside the sodium
iodide detectors or in front of them. Preferably, the array of
neutron detectors is located immediately in front of the sodium
iodide detectors.
The neutron detectors 88 yield a coarse image of the transparency
of the item to neutrons. The image contains both transmitted and
reflected neutrons. Blurring can be reduced by arranging
collimation around each detector so that it responds only to near
axis neutrons. The purpose of this detector array is to check for
regions of the item which neutrons do not penetrate thereby
revealing that neutron shielding has been employed. The use of
neutron absorbers would be recognised immediately as suspicious and
requiring further investigation. Gamma-rays from the neutron
absorbing materials are also detected by the germanium detector
86.
FIG. 6 illustrates a general signal processing system for the
number of detector output signals produced in the system. Basically
all signals are digitised and then processed by a computer. Power
supplies and high voltage detector bias circuits are not
illustrated in the drawing for clarity.
Referring first to the sodium iodide detectors of arrays 80,82,84
the output of each detector is passed via a pre-amp 90 to a sample
and hold circuit 92. For clarity not all detector output signal
leads are shown. Only the first and nth are indicated. The signal
values held in circuits 92 are periodically read and digitised by
analogue to digital convertor 94 and passed to a computer 96. As
previously discussed the amplitude of these signals is indicative
of gamma ray energy. Thus, by counting signals having amplitudes in
discrete narrow bands an energy distribution spectrum is built-up,
such as that shown as the lower curve in FIG. 5.
Signals from the single high resolution germanium detector 86 can
be built-up in the same way. Its output is passed through pre-amp
98 and sample and hold circuit 100. In the example this signal is
also sampled by A/D converter 94. Computer 96 notionally assembles
a high resolution energy spectrum such as the upper curve of FIG.
5.
In FIG. 5 the characteristic peaks of nitrogen, at 10.83 MeV, and
of hydrogen at 2.22 MeV can be clearly seen. Also the greater
resolution of the germanium detector is evident in the nitrogen
peak. Signals representing energy windows for specific gamma ray
energies are stored in a read only memory 102 and compared by
computer 96 with the counts at those energy levels in the energy
spectrum derived for an inspection item. In the event one or more
of the threshold levels is exceeded an alarm circuit 104 is
energised. In addition, or alternatively to an audible alarm, other
action may be taken. For example the alarm signal 106 may control
reject gate 9 in FIG. 1. Thus, when a suspect item is identified
the gate 9 may be lowered in synchronism with operation of the
conveyor 12 to divert the item to reject conveyor 8 for a second
level of inspection.
The output signals from the neutron radiography detectors 88 can be
processed in substantially the same way. Again only the first and
mth signal output lines are shown in the drawing. The signals are
passed through pre-amps 108 and sample and hold circuits 110 to an
analogue to digital converter 112. The computer 96 can then process
the digital signals to provide a coarse radiography image on screen
114. This may be combined with or superimposed on an x-ray image of
the item produced by x-ray apparatus 40.
The relatively modest shielding requirements of the arrangement
described above will be readily appreciated in view of the lack of
high energy neutrons. Conventional shielding materials may be
employed such as borax or dense boron loaded polyethylene.
Unfortunately this emits gamma-rays which will raise background
levels adversely affecting system sensitivity. Lithium carbonate
loaded polyethylene which produces greatly reduced gamma-ray
emmissions is preferred. For the same reason a moderator containing
heavy water (deuterium) is also preferred. The whole structure is
surrounded by lead to absorb stray emissions. The conveyers which
pass through the irradiation chambers are constructed of low gamma
emitting material and low residual activity material.
The invention has been described with reference to the particular
embodiment illustrated in the accompanying drawings. Nevertheless
it should be understood that various adaptations and modifications
may be made. The scope of the invention is limited only by the
following claims.
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